There is this guide: Audiobook Guide
But I’ll say Plex isn’t very well suited for audiobooks. As the guide suggests, you’ll need to use the Music library type. Plex added one feature, the “Store track progress” option, with audiobooks in mind, but the feature is only really useful if your audiobooks are in single huge files. If your books are in multiple files, Plex will remember its place in the track you’re playing, but it won’t remember which track that was.
